1. Field of the Invention
This invention relates to a diaphragm valve and more particularly to a high pressure packless metal diaphragm valve having a yoke member to valve body connection that facilitates the transfer of upward stem thrust from the valve stem to the valve body rather than to the bonnet when the valve is being closed under line pressures of substantial magnitude so that the bearing force exerted by the bonnet upon the diaphragm is not reduced and an effective seal is maintained around the diaphragm.
2. Description of the Prior Art
Conventionally known Y-diaphragm valves, as disclosed in U.S. Pat. Nos. 3,979,105, and 3,982,729, include a valve stem arranged for reciprocal movement in a yoke member of the valve. The yoke member may be threadedly secured, as illustrated in U.S. Pat. No. 3,979,105, to the upper end of a bonnet that extends axially into threaded engagement with the upper end of a valve body.
In the alternative, as disclosed in U.S. Pat. No. 3,982,729, the yoke member may be threadedly connected to the upper end of the valve body and extend into the upper end of the valve body chamber. With this arrangement the bonnet is concentrically positioned within the yoke member and valve body. The yoke member bears downwardly upon a flanged portion of the bonnet to, in turn, urge the bonnet into gripping engagement with the annular edge of a metallic flexible diaphragm. The diaphragm is supported at its lower surface upon an annular shoulder of the valve body.
In both of the above arrangements an enlarged head at the end of the valve stem bears upon the top surface of the diaphragm. The upper end of a valve member is resiliently supported within the lower portion of the valve body chamber and is maintained in abutting relation with the bottom surface of the diaphragm oppositely of the valve stem. The lower end portion of the valve member is positioned in overlying relation with a valve seat which connects inlet and outlet openings of the valve chamber.
A valve actuator is connected to the upper end of the valve stem. By rotation of the valve stem through the actuator the valve stem is movable upwardly and downwardly in the axially passageways of the yoke member and the bonnet. For example, upon downward movement of the valve stem, the diaphragm is downwardly deflected to transmit the downward movement to the valve member in the chamber lower portion. The spring assembly that maintains the valve member in a normally raised position from the valve seat is compressed by downward movement of the valve member and the valve member is moved into abutting, sealing relation with the valve seat to close the valve. Upward movement of the valve stem allows the spring assembly to raise the valve member off of the valve seat permitting fluid flow through the valve chamber between the inlet and outlet openings.
During the operation of the valve, to open or close the valve or to maintain the valve member in a preselected position between a closed or an opened position under full line pressure across the valve seat, the flexible diaphragm must be maintained in a position sealing the valve chamber lower portion from the valve chamber upper portion, particularly for valves handling contaminated fluids, such as radioactive fluids. To prevent leakage of fluid around the annular edge of the flexible diaphragm, the bonnet must apply a uniform downward bearing force upon the diaphragm annular edge. The bonnet must exert a bearing force required to maintain the diaphragm in sealed relation with the diaphragm seat under full line pressure.
A reduction in the bearing force applied to the diaphragm will result in leakage around the diaphragm. Diaphragm leakage is commonly experienced when the bearing force applied by the bonnet becomes unequally distributed on the annular edge of the diaphragm. If the bearing force is nonuniform, then the diaphragm will deflect in an irregular manner resulting in leakage around the diaphragm. It is well known that diaphragm leakage will eventually result in diaphragm failure by pressure blow-out of the diaphragm.
Diaphragms valves of the above type are commonly utilized to control the flow of contaminated fluids as encountered in nuclear power plants and the like. In this application of use, the diaphragm valve is frequently operated to maintain the valve member in a throttled position, i.e., in an intermediate position between an opened and closed position. When the valve member is maintained in a throttled position or is moved downwardly to engage the valve seat against full line pressure extreme upward thrust forces are exerted upon the valve member and transmitted to the valve stem. These upward thrust forces must be resisted in order to maintain the valve stem and the valve member in a preselected axial position relative to the valve seat and to prevent a reduction in the bearing force applied by the bonnet to the diaphragm.
In the known diaphragm valves the problem of transmitting the stem thrust from the valve stem to the bonnet and the yoke member is experienced. The upward stem thrust tends to relieve or reduce the downward bearing force applied by the bonnet upon the diaphragm. This condition can result in irregular deflection of the diaphragm leading to diaphragm leakage and eventual failure by blow-out of the diaphragm. Therefore, there is a need in Y-type diaphragm valves and particularly packless metal diaphragm valves, that are utilized to control fluid pressures requiring stem forces in excess of 100,000 lbs., to provide an internal valve structure that prevents the transmission of upward thrust forces from the valve stem to the bonnet and permits efficient replacement of the diaphragm assembly in the valve.
U.S. Pat. Nos. 2,382,235; 2,618,458; 2,812,777; 2,986,372; 3,955,794; and 3,874,636 also disclose diaphragm valves each having a bonnet externally threaded and engaging the internal threads of the valve body or housing to maintain the bonnet axially fixed in compressive relation with the flexible diaphragm. As discussed above this arrangement is also subject to a reduction in the seat force exerted by the bonnet upon the diaphragm by the transmission of upward thrust forces from the valve stem to the bonnet. In addition the arrangement of threadedly connecting the bonnet to the valve body is undesirable when the diaphragm is to be routinely replaced after a selected number of cycles of operation. The diaphragm can not be efficiently replaced if the bonnet must be unthreaded from its connection to the valve body. It is also known to encounter "freezing" of the bonnet to the valve body in a threaded connection. Thus to free the bonnet from the valve body in order to replace the diaphragm may require a substantial downtime for the valve and result in damage to the threads on the bonnet and valve body.
Of the above listed patents, U.S. Pat. Nos. 2,618,458; 3,874,636; and 3,955,794 disclose a connection of the yoke member to the bonnet whereby the bonnet is connected at an upper end to the yoke member and a lower end to the valve body. With this arrangement, the upward stem thrust is transmitted directly to the bonnet. The affect is a reduction in the bonnet bearing force upon the diaphragm. At high pressures this can result in loosening of the bonnet and leakage around the diaphragm.
U.S. Pat. Nos. 2,032,316 and 2,144,754 disclose a bolted connection between the bonnet and the valve body. The bonnet and the valve body include oppositely positioned radially extending flange portions provided with aligned bores for receiving a plurality of threaded fasteners or bolts. The flexible diaphragm is supported by a seat or ledge of the valve body. An external flange portion of the bonnet is positioned in overlying abutting relation with the annular edge of the diaphragm. The bonnet is secured to the valve body so that the bonnet is urged into compressive relation with the top surface of the diaphragm to exert a downward bearing force upon the diaphragm to maintain a fluid-tight seal between the bonnet and the valve body around the annular edge of the diaphragm.
In high pressure applications the valve stem may be subjected to a full line pressure resulting in upward thrust forces in excess of 100,000 lbs.. Consequently the bonnet must be securely bolted to the valve body to prevent a reduction in the bonnet bearing force and leakage around the diaphragm. This requires a substantial number of bolts arranged in a circular pattern on the bonnet. In order to accommodate the large number of bolts needed to generate the bearing force necessary to prevent leakage around the diaphragm when subjected to full line pressure, the bolt circle must have a substantial diameter.
When the upward stem thrust is transmitted to the bonnet, the bonnet bolts are subjected to linear deflection. U.S. Pat. No. 2,144,754 is an example of a diaphragm valve having a relatively large diameter bonnet bolt circle in order to accommodate a large number of bonnet bolts. Linear deflection of the bolts resulting from upward stem thrust loosens the bonnet bolts. Consequently, a non-uniform bearing force is applied by the bonnet and leakage develops around the diaphragm because the diaphragm is deflected irregularly around the outer edge thereof.
The greater the bonnet circle, the greater the number of bolts that are required to connect the bonnet to the valve body. This arrangement increases the difficulty of maintaining a constant and uniform bearing force upon the outer annular edge of the diaphragm. Thus for diaphragm valves having a relatively large bonnet bolt circle difficulty is frequently experienced in maintaining an effective seal around the diaphragm. Because a large bearing force of a constant magnitude is required any deviation in the forces generated by the bonnet bolts result in deflection of the diaphragm and diaphragm leakage.
While it has been suggested by the prior art diaphragm valves to secure the bonnet to the valve body and/or to the yoke member in a manner to maintain the bonnet in compressive relation with the diaphragm to prevent leakage around the diaphragm, the known valves are subject to diaphragm leakage by the transmission of upward stem thrust from the valve stem to the bonnet. Also the known arrangements do not facilitate efficient replacement of the diaphragm in high pressure diaphragm valves.
Therefore, there is need for a large high pressure packless metal diaphragm valve having internal valve structure that prevents the transmission of stem thrust to the bonnet so as to prevent a reduction in the bearing force upon the diaphragm. There is also need in high pressure diaphragm valves having the bonnet bolted to the valve body to reduce the diameter of the bonnet bolt circle so as to reduce the number of bolts that connect the bonnet to the valve body to maintain the bearing force equally distributed around the outer annular edge of the diaphragm.